1. Field of the Invention
This invention relates to a steam turbine for use in nuclear power plants, geothermal power plants and so forth, and more particularly to steam turbine designed so as to prevent stress corrosion cracking of shrink-fit type turbine rotors.
2. Description of the Prior Art
In general, the rotors of steam turbines for use in nuclear power plants, geothermal power plants and the like may be manufactured by processes as follows: machining after monobloc forging from material such as alloy steel, machining after monobloc welding of disc-shaped materials to the rotor shaft, and fitting wheel discs which have previously been machined and provided with implanted blades, to the rotor shaft so as to form a monobloc in shrink-fit manner. Of these types, the shrink-fit type turbine rotor has been used for many years because larger-size turbine rotors can be manufactured from relatively small forging materials, since the rotor shaft and respective disc plates are initially separate.
FIG. 1 shows one example of the conventional shrink-fit type rotor. In FIG. 1, the inner diameters of respective wheel discs 2 are manufactured to be smaller in size than corresponding outer diameters of a rotor shaft 1 by an amount known as a shrinkage allowance at room temperatures. In order to shrink fit the wheel discs 2 onto the shaft 1, the wheel discs 2 are first heated, while not raising the temperature of the rotor shaft 1, so that the wheel disks 2 thermally expand. After the inner diameters of the wheel discs 2 are expanded to such an extent as to be larger than the corresponding outer diameters of the shaft 1, they are positioned on the shaft 1 and are disposed thereon at specified positions. Thereafter the wheel discs 2 are cooled down and shrink fitted onto the shaft 1 by virtue of thermal shrinkage so that a turbine rotor results. On the outer circumferences of the respective wheel discs 2 which are thus rigidly fixed, there are implanted plural blades 3 so as to constitute turbine blades.
The respective connecting faces between the rotor shaft 1 and the wheel discs 2 are respectively provided with key ways 4 and 5 within which bore keys 6 are disposed. The bore keys 6 serve to prevent the respective wheel discs 2 from relatively rotating with respect to the shaft 1, even when the shrink-fit is unintentionally loosened under abnormal operating conditions.
However, in a steam turbine plant using such a shrink-fit type turbine rotor, the turbine rotor is in danger of developing so-called stress corrosion cracks with the result that the turbine rotor deteriorates in reliability and the life span thereof is shortened. One aspect of the development mechanism of stress corrosion cracking is a phenomenon such that in the environment of water or steam containing oxygen, metal surface oxide films are locally destroyed and selectively restructured due to the tensile stress effected on the material thereof, resulting in development of the cracks. Accordingly, the stress corrosion cracks develop upon concurrent occurrence of three factors as follows: that the material has sensitivity with respect to cracking, that high stress of a value more than a specified limit value is effected, and that the material is situated in an environment in which the material undergoes both development and destruction of local oxide films.
The sensitivity of the material to stress corrosion cracking is also related to material strength and, in general, the higher the tensile strength of the material the higher its sensitivity to the cracking. The wheel discs 2 of the shrink-fit type turbine rotor inevitably require, due to the higher stress effected thereupon, the use of higher tensile-strength alloy steels, and it is not forseeable that materials with no sensitivity to such cracking could be developed in the future.
When considering the stress effected upon the wheel discs 2 of the shrink-fit type turbine rotor, it is recognized that there are developed both the shrink-fit stress derived from the intial stage shrink-fit and the centrifugal stress derived from the fact that centrifugal force acts on both the wheel discs 2 and the blades 3 during rotation. The values of such stress become higher at the radially inner portions of the wheel discs 2. In particular, on the key ways 5 around the disc bore keys 6, there are developed stress concentrations derived as a function of the configurations thereof, and the values of such stresses frequently exceed the limit value beyond which stress corrosion cracks are initiated.
Moreover, in terms of environment, the qualitative characteristiscs of steam in power plant facilities are determined on the basis of the overall specifications of such facilities as a steam generator (boiler, nuclear reactor and the like), a condenser and a feed water facility, so that it is difficult to carry out a sophisticated water quality control program concentrated only upon concern for the stress corrosion cracking of the wheel discs 2. Particularly in a BWR (Boiling Water Reactor) type nuclear power plant or a geothermal power plant, it is impossible to prevent oxygen (impurities) generated within the nuclear reactor from penetrating into the steam turbine together with steam.
In the vicinity of the key ways 5 of the wheel discs 2 of the above-described conventional shrink-fit type rotor, there is a danger of development of stress corrosion cracks because the three factors mentioned above are present. If the stress corrosion cracks develop on the wheel discs 2 and are not discovered early enough to be nipped in the bud by means of test procedures such as NDI (Non-Destructive Investigation), this can lead to the destruction of the wheel discs 2, resulting in the danger of serious accidents.
As a countermeasure with respect to the environment, which is one of the factors affecting development of such stress corrosion cracking, attempts have been made to maintain the environment such that clean steam with less impurities is supplied from outside the steam turbine to the wheel discs 2 which are in danger of developing the stress corrosion cracks so as to cause the wheel discs and the vicinity thereof to be less susceptible to such stress corrosion cracking. However, to achieve this it is necessary to independently install a steam supply source that generates clean dry steam.
A nuclear power plant which incorporates a steam turbine that uses the conventional shrink-fit type turbine rotor is constituted as shown in FIG. 3, wherein steam from a nuclear reactor 8 is led through a high pressure steam turbine 9 via a main steam line into a low pressure steam turbine 10 so as to become spent. The spent steam is then led to a condenser (not shown).
Condensate from the condenser is, in turn, boosted in pressure by means of a feed water pump (not shown) and fed through a filter into a feed-water heater 11. Within the feedwater heater, the condensate is heated by turbine extraction steam supplied from the low pressure turbine 10 or the high pressure turbine 9 through an extraction steam pipe 12. The thus heated condensate is returned by means of a feed-water pump (not shown) into the nuclear reactor 8.
In the conventional nuclear power plant, there is provided a gland seal steam supply apparatus 13 that supplies gland seal steam into the high pressure turbine 9 and the low pressure turbine 10. The gland seal steam supply apparatus 13 includes an evaporator 14 into which clean condensate with low impurities from a condensate storage tank 15 is supplied through a feed-water pump 16. The condensate supplied into the evaporator 14 is heated either by main steam which is obtained from the nuclear reactor 8 and reduced in pressure through a main steam control valve 17 or by turbine extraction steam from the low pressure turbine 10.
The steam pressure required for gland sealing of the turbines 9 and 10 is relatively low, for instance, as low as 0.28 kg/cm.sup.2 so that the pressure of the steam generated within the evaporator 14 can be as low as approximately 3 to 4 kg/cm.sup.2 at most, and the steam can be saturated steam with a wetness of approximately 1%. The pressure of the steam generated within the evaporator 14 is so low that the pressure of the condensate heating steam (medium) can also be low. Thus, the condensate heating steam can be extraction steam which is extracted from an intermediate stage of the low pressure turbine 10.
Hitherto, however, the gland seal steam supply apparatus 13 has served only to supply the low pressure steam generated within the evaporator 14 into the gland seal portions of the high pressure turbine 9 and the low pressure turbine 10 for gland sealing, and any other use of such steam has never been considered.